Compliant foil fluid film radial bearings are currently being utilized in a variety of high speed rotor applications. These bearings are generally comprised of a bushing, a rotating element such as a rotor or shaft adapted to rotate within the bushing, non-rotating compliant fluid foil members mounted within the bushing and enclosing the rotating element, and non-rotating compliant spring foil members mounted within the bushing underneath the non-rotating compliant fluid foil members. The space between the rotating element and the bushing is filled with fluid (usually air) which envelops the foils.
The motion of the rotating element applies viscous drag forces to the fluid in the converging wedge channels. This results in increases in fluid pressure, especially near the trailing end of the wedge channels. If the rotating element moves toward the non-rotating element, the convergence angle of the wedge channel increases, causing the fluid pressure rise along the channel to increase. Conversely, if the rotating element moves away, the pressure rise along the wedge channel decreases. Thus, the fluid in the wedge channels exerts restoring forces on the rotating element that vary with and stabilize running clearances and prevent contact between the rotating and non-rotating elements of the bearing. Flexing and sliding of the foils causes coulomb damping of any axial or overturning motion of the rotating element of the bearing.
Owing to preload spring forces or gravity forces, the rotating element of the bearing is typically in physical contact with the fluid foil members of the bearing at zero or low rotational speeds. This physical contact results in bearing wear. It is only when the rotor speed is above what is termed the lift-off/touch-down speed that the fluid dynamic forces generated in the wedge channels assure a running gap between the rotating and non-rotating elements.
Compliant foil fluid film radial bearings typically rely on backing springs for preload, stiffness, and damping. The fluid foils are preloaded against the relatively movable rotating element so as to control foil position/nesting and to establish dynamic stability. The bearing starting torque (which should ideally be zero) is directly proportional to these preload forces. These preload forces also significantly increase the rotor speed at which the hydrodynamic effects in the wedge channels are strong enough to lift the rotating element of the bearing out of physical contact with the non-rotating members of the bearing. These preload forces and the high liftoff/touch-down speeds result in significant bearing wear each time the rotor is started or stopped.
Conventional compliant foil fluid film radial bearings operate with extremely small running clearances and moderate, as opposed to low, drag and power consumption. The clearances between the non-rotating fluid foil's minimum film region and the rotating element are typically less than 100 micro-inches at operating conditions.
In some instances, the compliant fluid foil elements may comprise a plurality of individual compliant foils to form a plurality of wedge shaped channels which converge in thickness in the direction of the rotation of the rotor, while in other instances the compliant foils may be formed on a single sheet enveloping the rotor within a cylindrical or lobed bushing.
One of the high speed applications in which these compliant foil fluid film radial bearings are utilized is a gas turbine engine. Such an engine could include a gas turbine and a gas compressor at opposite ends of a shaft with the compliant foil fluid film radial bearing in between. Since hot combustion gases, as high as 1,750 degrees Fahrenheit are expanded in the gas turbine and atmospheric air is compressed in the compressor, there is a high degree of temperature variation between the turbine or hot end of the compliant foil fluid film radial bearing and the compressor or cooler end of the compliant foil fluid film radial bearing. In some instances the hot end of the compliant foil fluid film radial bearing will be as much as 500 degrees Fahrenheit hotter than the cooler end of the compliant foil fluid film radial bearing.
In most cases, the compliant foil fluid film radial bearing will be generally uniform from the turbine end to the compressor end and the bushing and rotor will also be a constant diameter. As a result of the temperature gradient from the hot end to the cooler end of the compliant foil fluid film radial bearing, the bearing's radial play (sway space or running clearance) may be severely restricted at the turbine end of the bearing. The restriction in radial play, the result of greater thermal expansion at the turbine end of the bearing, can cause the bearing to lose load capacity, have an excessively high touch-down speed, have excessive drag, have excessive power consumption, or be forced to operate at an unacceptable temperature due to self heating. All of the above will adversely impact bearing life and reliability.
U.S. Pat. Nos. 4,502,795 issued Mar. 5, 1985, and 4,555,187 issued Nov. 26, 1985, both entitled Foil Bearing Alignment, propose the use of shims between the compliant foil fluid film radial bearing and the axially extending bore to radially outwardly diverge the surface of the bearing along its axial length. Alternately, an axially varying underspring rib thickness is proposed as is a conical bearing bushing to correct or minimize bearing rotor misalignment and accommodate bearing rotor deflection or tilt.